Wednesday, February 1, 2017

A graphical explanation of how the Sierra Nevada got Januburied

This was Januburied (below). If you are coming from TGR, welcome to our little forecast site for the Sierra Nevada, and if you have been following us for the past five years, thanks for your support!

The figure above (and again a bit below to avoid you scrolling constantly) demonstrates how it all happened. In short, multiple landfalling atmospheric rivers (ARs) brought substantial precipitation to the Sierra Nevada throughout much of January. There were some scary times for the snowpack and regarding flooding on either side of the Sierra (notably January 7-8 and the time leading up to that event when we knew we were in for a big one but the outcome was uncertain). The one-two punch of a colder storm (AR #1) followed by a warmer AR #2 (note high snow levels around 8500-9500 ft) is a 3 Michelin Star recipe for rain-on-snow induced flooding. Following these storms, we got extremely lucky with the consistently cooler temperatures and lower snow levels that produced copious snowfall and low elevations during ARs #3-5.

Let's break this down a bit. An excellent metric that has been developed to identify atmospheric rivers, which are long (2000 km), narrow (typically less than 1000 km), corridors of enhanced precipitable water (blue dots; pwater or pdub for you hip hop fans) and water vapor transport (wind times precipitable water). They (ARs) exist in the warm sector of cyclones, hence their often high snow levels. Note how the black dots below tend to be higher during AR conditions (green boxes, when observed pwater exceeds 20 mm along the California Coast). ARs are a form of a low level jet (strong winds), which when these strong winds slam into terrain, produce incredible precipitation totals over multi-day periods. In fact, California sees some of the largest 3-day total precipitation magnitudes in the United States, on par with hurricane precipitation totals in the southeastern U.S.

The narrow plumes of high precipitable water are known as atmospheric rivers. This view from the SSM/I instrument shows AR#2 and #3 making landfall. The start of Januburied was produced by the persistent counterclockwise circulation in the northeastern Pacific (low pressure) that favored continuous atmospheric river landfalls. In California, we receive nearly 50% of our annual precipitation from these types of storms and they contribute to nearly all of the variability of our precipitation. In other words, a handful of these, or lack thereof, make the difference between an awesome wet year and a crappy drought year.

Next, note how significant precipitation (blue bars) was observed during these events along the western slope of the Sierra (Blue Canyon, one of the wetter spots in the northern central Sierra Nevada). The peaks and valleys of the precipitation (bars) line up beautifully with the peaks in pwater (blue dots).The grey line shows daily Donner Summit snow depth (all other data is hourly) at en elevation of approximately 6900 ft (the Central Sierra Snow Lab for those of you familiar with SNOTEL or the Summit itself). During all ARs, with the exception of AR #2, substantial increases in snow depth occurred, particularly during the AR #3 (the official start of Januburied?).

The slow decline in snow depth during non-precipitating conditions is likely due to settling of snow.

Had I plotted snow water equivalent, or the amount of water stored in the snowpack, the line would just launch upwards in all cases. This concept is where we got lucky again (at least I think so). AR #1, though somewhat of an upside down storm (snow levels rose through the first part, leading to more dense snow being deposited atop less dense snow), gave us a nice snowy sponge to absorb much of the rainfall that came during the flood event (AR #2). This massively boosted the water stored in the snowpack (very good for water resources and the destruction of certain sneaky persistent weak layers). Had we not had this pre-storm boost, who knows what the outcome may have been!

The subsequent storms entrained much more cold air and had lower snow levels, especially during ARs #4-5, which was the icing on the already triple layer cake as we went from 'some snow down low' to 'oh yea, the 5000-9000(!) foot crest-to-valley lines go!'

To make things even more awesome, when it wasn't snowing, it was cold and/or cloudy (sorry, I could have plotted solar radiation but I got lazy at the end), and we avoided the wicked wind of the east, with a six hour exception. This favored extremely good snow conditions from summits to the lowest elevations, and created much rejoicing and probably got more than a few people (myself included) a bit behind in work.

Dr. Hatchett researching snow transport.

Dr. Morales engages in some intradisciplinary snow science studies. One of the best bonuses of atmospheric rivers and spillover precipitation are that they help fill up desert terminal lakes such as Washoe Lake in the background. These lakes are natural rain gauges and make for cool photographs.

The data used in the Januburied figure is publicly available and was acquired from NOAA, UCAR, CDEC, and XMACIS.